Quote from: pxstein on Today at 07:21:23 AM

But why are there yellow AND green empty bitcoin addresses?

As nc50lc has said, the yellow ones are change addresses.

When you receive bitcoin from an outside source, Electrum will only show green "receiving" addresses on the "receive" tab.
When you receive change back from your own transaction, Electrum will only send it to a yellow "change" address.
Note that there is no inherent difference between how the addresses work, and you could receive external bitcoin to a change address or send your own bitcoin back to a receiving address if you wanted, but Electrum would never do this automatically.

Quote from: pxstein on Today at 07:21:23 AM

As far as I understand if I send some 0.02 BC from an address in my Electrum Wallet containing 0.05 BC then Electrum
- sends 0.02 to the given target address of the recipient
- creates automatically a new empty address
- send the remaining change of 0.03 BC to this new address
- add the new, just created address automatically to the list of yellow change addresses visible on the "Adresses" tab.

Not quite. Rather than creating a new empty change address, it will simply pick the next unused change address from the list it pregenerates, and send your chance there. As you start using these change addresses, Electrum will automatically generate more to keep a list of 10 unused ones as default.

Quote from: pxstein on Today at 07:21:23 AM

So why are there EMPTY (!) yellow addresses listed on "Addresses" Tab?

As above. By default Electrum generates a list of 20 unused receiving addresses and 10 unused change addresses. When you use an address at the top of the list, it creates a fresh unused one at the bottom of the list.